1. Field of the Invention
The present invention relates to a cation exchange membrane of a fluorinated polymer for electrolysis. More particularly, it relates to a cation exchange membrane of a fluorinated polymer having excellent electrochemical properties and mechanical properties which is suitable as a membrane for electrolysis of an aqueous solution of an alkali metal chloride.
2. Description of the Prior Art
Recently, it has been proposed to use a cation exchange membrane for electrolytic production of an alkali metal hydroxide and chlorine in high purity instead of a conventional asbestos diaphragm.
It has been known that a membrane of a fluorinated polymer has desirable characteristics as the cation exchange membrane because of the requirements of oxidation resistance, chlorine resistance, alkaline resistance and heat resistance.
The cation exchange membranes made of a copolymer of a fluorinated olefin and a fluorinated monomer having a sulfonic acid group or a functional group which can be converted to a sulfonic acid group such as the copolymer of CF.dbd.CF.sub.2 and CF.sub.2 .dbd.CFOCF.sub.2 CF(CF.sub.3)O(CF.sub.2).sub.2 SO.sub.2 F or a copolymer of a fluorinated olefin and a fluorinated monomer having a carboxylic acid group or a functional group which can be converted to carboxylic acid group such as the copolymer of CF.sub.2 .dbd.CF.sub.2 and CF.sub.2 .dbd.CFO(CF.sub.2).sub.3 COOCH.sub.3 and the copolymer of CF.sub.2 .dbd.CF.sub.2 and CF.sub.2 .dbd.CFOCF.sub.2 CF(CF.sub.3)O(CF.sub.2).sub.3 COOCH.sub.3 have been known as membranes having good electrolytic characteristics.
The latter type of cation exchange membranes prepared from the fluorinated polymer having carboxylic acid groups as the cation exchange groups has been considered to be superior to the former type of cation exchange membranes prepared from the fluorinated polymer having sulfonic acid groups as the cation exchange groups when used as the cation exchange membrane for electrolysis because the electrolysis can be performed at high current efficiency with low electrolytic voltage by using the latter even if an alkali metal hydroxide having high concentration is produced.
For example, an alkali metal hydroxide having a concentration higher than 40 wt. % can be produced in at a current efficiency higher than 90% by the electrolysis of an alkali metal chloride using said cation exchange membrane having carboxylic acid groups.
However, when the cation exchange membrane of the fluorinated polymer having carboxylic acid groups as the cation exchange groups is used for the electrolysis, there is a disadvantage of weak strength against various mechanical shocks occurring during the electrolysis though it has excellent electrochemical characteristics.
The cation exchange membrane contacts an alkali metal hydroxide in high concentration on one side and contacts an alkali metal chloride on the other side in the electrolysis. Moreover, the different pressures of the different gases are respectively applied to the membrane, whereby the phenomena of partial swelling and contraction of the membrane are caused. Furthermore, hardening of the membrane in ageing may occur whereby the creases, cracks and laminal peeling of the membrane may be caused to form pin holes which cause leakage of the raw material of alkali metal chloride into the alkali metal hydroxide and a decrease of the current efficiency.
Heretofore, in order to improve the mechanical strength of the cation exchange membrane, it has been known to reinforce the membrane by disposing a reinforcing material such as fabric or non-woven fabric of polypropylene, polyvinyl chloride, polyvinylidene fluoride, or polytetrafluoroethylene in the membrane.
In this way, the dimensional stability of the membrane is improved; however the strength against the severe mechanical shocks encountered during the electrolysis is not satisfactorily improved.
In said reinforcement, the electric characteristics such as the increase of electric resistance of the cation exchange membrane are deteriorated and the reinforcing material is expensive.